Manufacturers of rechargeable batteries give recommended charge rates and times for bringing a specified battery up to 100% of its rated capacity from a condition of zero charge. However, batteries in actual use are quite often recharged from a condition of incomplete discharge. The educated user is able to adjust the charging time for such batteries to achieve close to a 100% charge condition. More often than not, however, the user overcharges such batteries by following what he considers to be the manufacturer's recommended charging procedure.
Devices have been used, typically for automobiles, for recharging batteries in a manner that prevents a greater than 100% charge condition. These devices operate on the principle of sensing the terminal voltage of the battery, reducing the charging current when the terminal voltage exceeds a predetermined value. When this type of device is used with nickel cadmium cells or the like which have negative temperature coefficients, a condition of overcharge is often realized because of heating of the cells from the charging process and/or changes in ambient conditions. Some charging devices have been constructed which take into account the negative temperature coefficient of nickel cadmium cells or the like but are lacking a fine degree of control of the charging process.